Worms in Horses: Faecal Egg Counts, Deworming and Management

Worms are present in every horse, but when a horse suffers from a worm infection, this can lead to health issues. Worms are not visible in the faeces, so it is not possible to determine whether, and to what extent, a horse is infected without testing. That is why it is important to carry out regular faecal egg counts (FECs) and, when necessary, use appropriate deworming treatments.​

Worms are present in every horse, but when a horse suffers from a worm infection, this can lead to health issues. Worms are not visible in the faeces, so it is not possible to determine whether, and to what extent, a horse is infected without testing. That is why it is important to carry out regular faecal egg counts (FECs) and, when necessary, use appropriate deworming treatments.​



What are worms and which types affect horses?

Worms are parasites that can only survive at the expense of another organism, in this case, the horse. In horses, worms are primarily found in the digestive system. Most horses carry worms, but typically, the presence of a few worms doesn’t cause any problems. However, if a horse suffers from a more severe worm burden, it can lead to a range of health issues¹. As worms mainly inhabit the digestive tract, their eggs are excreted in the horse’s faeces.

Several different types of worms can infect horses. The most common include tapeworms, threadworms, roundworms, large strongyles, lungworms and small redworms².

Resistance and Faecal Egg Counts 

Because worm eggs are excreted in the faeces, faecal egg counts can be carried out to determine whether a horse has a worm infection and how severe it is. FECs can also help identify which type of worm is present. Although faecal testing can be performed year round, it is particularly important to carry out testing during the spring and summer months, as research has shown this is when egg shedding is at its peak³.

Faecal testing is becoming more common and is increasingly preferred over routine deworming schedules. More and more, horse owners are choosing to carry out a faecal egg count first to determine whether deworming is necessary. This targeted approach ensures that only horses with a high egg count are treated, helping to reduce unnecessary use of wormers.

Frequent and routine use of dewormers, without knowing whether they are actually needed, can lead to resistance⁴. Resistance means that worms become less sensitive to the active ingredients in the wormers used, therefore making treatment less effective in the event of an infection. When a dewormer is given, resistant worms will survive and remain in the horse⁵. Resistance to certain wormers has already been observed. For this reason, it is strongly recommended to carry out a faecal egg count before deworming in order to prevent further resistance. Additionally, targeted deworming based on testing can also help reduce costs.

Management

There are several management measures that can help reduce the risk of worm infection, including maintaining good hygiene in the stable and fields. Effective pasture management is one of the most important ways to prevent worm infestations. 

For example, ensure that faeces are removed from fields regularly. Worm infestations spread more easily when droppings from infected horses are left in the field, allowing other grazing horses to ingest the worms.​

If possible, rotate horses onto clean pastures regularly. This allows the previously grazed field to rest, giving the horses access to fresh fields where the risk of infection is lower. Before moving horses onto clean pasture, it is recommended to carry out a faecal egg count to help keep the new field as clean as possible.

Infections can also be reduced by grazing fields with other species, such as sheep. These animals are not susceptible to the same types of worms as horses and will consume the worm larvae, effectively ‘cleaning’ the pasture. Additionally, horses cannot be infected by worm eggs shed by sheep.

To prevent resistance and minimise the risk of infection, it is therefore important to carry out regular faecal egg counts and maintain clean pastures. Testing is especially important in spring and summer, when egg shedding is at its highest. Good pasture management and hygiene play a key role in effective worm control.

 

References

1.           Saeed, M.A., Beveridge, I., Abbas, G., Beasley, A.M. (2019) Systematic review of gastrointestinal nematodes of horses from Australia. Parasites and Vectors, 12(188):1-16.

2.           Bulgaru, A., Tudor, P. (2015) The Prevalence of Helminth Parasites in Horses Raised in Modern Conditions. Scientidic Works Series C Veterinary Medicine, 2:271-276.

3.           Wood, E.L.D., Matthews, J.B., Stephenson, S., Slote, M., Nussey, D.H. (2013) Variation in fecal egg counts in horses managed for conservation purposes: individual egg shedding consistency, age effects and seasonal variation. Parasitology, 140(1):115-128.

4.           Tzelos, T., Matthews, J. (2016) Anthelmintic resistance in equine helminths and mitigating its effects. In Practice, 38:489-499.

5.           Metthews, J.B. (2022) Anthelmintic resistance in equine nematodes. International Journal of Parasitology Drugs and Drug Resistance, 4:310-315.






What are worms and which types affect horses?

Worms are parasites that can only survive at the expense of another organism, in this case, the horse. In horses, worms are primarily found in the digestive system. Most horses carry worms, but typically, the presence of a few worms doesn’t cause any problems. However, if a horse suffers from a more severe worm burden, it can lead to a range of health issues¹. As worms mainly inhabit the digestive tract, their eggs are excreted in the horse’s faeces.

Several different types of worms can infect horses. The most common include tapeworms, threadworms, roundworms, large strongyles, lungworms and small redworms².

Resistance and Faecal Egg Counts 

Because worm eggs are excreted in the faeces, faecal egg counts can be carried out to determine whether a horse has a worm infection and how severe it is. FECs can also help identify which type of worm is present. Although faecal testing can be performed year round, it is particularly important to carry out testing during the spring and summer months, as research has shown this is when egg shedding is at its peak³.

Faecal testing is becoming more common and is increasingly preferred over routine deworming schedules. More and more, horse owners are choosing to carry out a faecal egg count first to determine whether deworming is necessary. This targeted approach ensures that only horses with a high egg count are treated, helping to reduce unnecessary use of wormers.

Frequent and routine use of dewormers, without knowing whether they are actually needed, can lead to resistance⁴. Resistance means that worms become less sensitive to the active ingredients in the wormers used, therefore making treatment less effective in the event of an infection. When a dewormer is given, resistant worms will survive and remain in the horse⁵. Resistance to certain wormers has already been observed. For this reason, it is strongly recommended to carry out a faecal egg count before deworming in order to prevent further resistance. Additionally, targeted deworming based on testing can also help reduce costs.

Management

There are several management measures that can help reduce the risk of worm infection, including maintaining good hygiene in the stable and fields. Effective pasture management is one of the most important ways to prevent worm infestations. 

For example, ensure that faeces are removed from fields regularly. Worm infestations spread more easily when droppings from infected horses are left in the field, allowing other grazing horses to ingest the worms.​

If possible, rotate horses onto clean pastures regularly. This allows the previously grazed field to rest, giving the horses access to fresh fields where the risk of infection is lower. Before moving horses onto clean pasture, it is recommended to carry out a faecal egg count to help keep the new field as clean as possible.

Infections can also be reduced by grazing fields with other species, such as sheep. These animals are not susceptible to the same types of worms as horses and will consume the worm larvae, effectively ‘cleaning’ the pasture. Additionally, horses cannot be infected by worm eggs shed by sheep.

To prevent resistance and minimise the risk of infection, it is therefore important to carry out regular faecal egg counts and maintain clean pastures. Testing is especially important in spring and summer, when egg shedding is at its highest. Good pasture management and hygiene play a key role in effective worm control.

 

References

1.           Saeed, M.A., Beveridge, I., Abbas, G., Beasley, A.M. (2019) Systematic review of gastrointestinal nematodes of horses from Australia. Parasites and Vectors, 12(188):1-16.

2.           Bulgaru, A., Tudor, P. (2015) The Prevalence of Helminth Parasites in Horses Raised in Modern Conditions. Scientidic Works Series C Veterinary Medicine, 2:271-276.

3.           Wood, E.L.D., Matthews, J.B., Stephenson, S., Slote, M., Nussey, D.H. (2013) Variation in fecal egg counts in horses managed for conservation purposes: individual egg shedding consistency, age effects and seasonal variation. Parasitology, 140(1):115-128.

4.           Tzelos, T., Matthews, J. (2016) Anthelmintic resistance in equine helminths and mitigating its effects. In Practice, 38:489-499.

5.           Metthews, J.B. (2022) Anthelmintic resistance in equine nematodes. International Journal of Parasitology Drugs and Drug Resistance, 4:310-315.